Neuromuscular junctions of the posterior cricoarytenoid muscle in the adult rat: a scanning electron microscopical study.

Neuromuscular junctions in the posterior cricoarytenoid (PCA) muscle of the adult rat larynx were examined by scanning electron microscopy. Two types of focal junctions were characterized by the nerve endings with terminal varicosities or terminal buttons and by the subneural apparatuses with a group of cup-like depressions (2-5 microns in diameter), not labyrinthine gutters. One type of the subneural apparatuses consisted of a large number (more than 20) of depressions with predominantly slit-like junctional folds. The other type was characterized by a small number (about 10) of depressions with a few junctional folds. From the relative proportions of the two types of muscle fibers and subneural apparatuses in the rat PCA muscle, we suggest that the Type II fibers have the apparatus consisting of numerous small depressions with many junctional folds, while that of the Type I fibers consists of a small number of depressions with a smaller number of junctional folds. It is suggested that the structural differences of the subneural apparatus reflect different types of muscle fibers.